Hot Weather Concreting(1).pptx

Hot Weather Concreting

1. HOT WEATHER CONCRETING

2. Relevance Huge volume of concreting is in practice at several different geographical locations and different weathers like normal weather, hot weather and in cold weather conditions. Hence it becomes necessary to adopt proper concreting methods to get the best results with in available resources. With reference to the Indian weather scenario, the weather conditions are very much different in different parts of India and hence from Hot weather conditions point of view it’s a relevant topic to discuss for concreting.

3. Definition (SP:23 – 1982, Section 7.1, Page No.125) Hot Weather Concreting : Any Operation of concreting done at the atmospheric temperature beyond 40 C or any operation of concreting (Other than Steam curing) where the temperature of concrete during it’s placement is expected to be more than 40 C may be put under Hot weather concreting. Concrete is not recommended to be placed at or beyond the temperature 40 C without proper precautions IS : 7861 (Part 1) -1981 (Hot Weather Concreting)

4. ISSUES Typical Problems are encountered in the preparation, placement & curing of concrete in hot weather. High temperatures results in Rapid hydration of cement Increased evaporation of mixing water Greater mixing water demand Large volume changes resulting in cracks The problems of hot weather concrete are further aggravated by a number of factors, such as use of rapid hardening cements, handling of larger batches of concrete, etc.

5. The climatic factors affecting concrete in hot weather are High ambient temperature Reduced relative humidity The effects of these may be considerably more pronounced with increase in wind velocity. The effects of hot weather are most critical during periods of Rising temperature Falling relative humidity Both Precautionary measures required on a calm, humid day will be less strict than those required on a dry, windy day even if air temperatures are identical. ISSUES

6. EFFECTS OF HOT WEATHER ON CONCRETE

7. EFFECTS OF HOT WEATHER ON CONCRETE Accelerated Setting Increases the rate of setting of the concrete. The concrete handling time is reduced. Quick stiffening may necessitate undesirable re-tempering of concrete by addition of water. It may also results in cold joints Reduction in Strength High temperature increases the quantity of mixing water to maintain the workability with consequent reduction in strength. Addition of plasticizers adds to the cost.

8. EFFECTS OF HOT WEATHER ON CONCRETE Increased Tendency to Crack Plastic shrinkage cracks may form. Partially in the hardened concrete due to rapid evaporation of water. Cracks may be developed in hardened concrete either by; Increased drying shrinkage resulting from greater mixing water used. By cooling of the concrete from its elevated initial temperature.

9. EFFECTS OF HOT WEATHER ON CONCRETE PROPERTIES

10. EFFECTS OF HOT WEATHER ON CONCRETE Rapid Evaporation of Water During Curing Period It is difficult to Retain moisture for hydration and Maintain reasonably uniform temperature conditions during the curing period. Difficulty in Control of Air Content in Air – Entrained Concrete It is more difficult to control air content in air – entrained concrete This adds to the difficulty of controlling workability For a given amount of air entraining agent, hot concrete will entrain less air than concrete at normal temperatures.

11. EFFECTS OF HOT WEATHER ON CONCRETE PROPERTIES The properties of concrete, for example, strength, impermeability; dimensional stability; resistance to weathering, wear and chemical attack can be adversely affected by combinations of high air temperature, low relative humidity and high wind velocity.

12. EFFECT ON COMPRESSIVE STRENGTH Concretes mixed , placed and cured at elevated temperatures normally develop higher early strength. But at 28 days or later the strengths are lower. Further reduction in strength can occur if sufficient water curing is absent or there is considerable delay in the commencement of moist curing.

13. EFFECTS ON WORKABILITY AND WATER DEMAND

14. EFFECTS ON WORKABILITY AND WATER DEMAND For maintaining the same workability, the quantity of water in the concrete has to be increased as the concrete temperature increases. If the amount of water remains unchanged, there will be loss of workability.

15. EFFECTS ON WORKABILITY AND WATER DEMAND The dotted line illustrates the effects of increasing concrete temperature on the resulting workability of concrete when the amount of net mixing water is held constant. It indicates that an increase of 11 o C in temperature may be expected to decrease the slump by 25 mm.

16. EFFECTS ON WORKABILITY AND WATER DEMAND The solid line illustrates change in water requirement that may be necessary to produce a 25mm change in slump at various temperatures levels. As the temperature increases from 5 o C to 27 o C, only 2.25 to 2.5% additional water will be required to effect a 25 mm change in slump, while 4.5% would be required when the temperature reaches 49 o C

17. EFFECTS ON SHRINKAGE

18. EFFECTS ON SHRINKAGE Plastic shrinkage In the hot weather, whenever the rate of evaporation of water from the concrete mix is greater than the rate at which water rises to the surface of freshly placed concrete (bleeding), plastic shrinkage cracking will usually occur. High concrete temperature, high wind temperature, high wind velocity and low relative humidity, or combinations thereof, cause rapid evaporation, which significantly increases the likelihood of occurrence of plastic shrinkage cracking. Whenever the rate of evaporation is estimated to be approaching 1.0 kg/m 2 /hr, necessary precautions against plastic shrinkage concrete should be taken.

19. EFFECTS ON SHRINKAGE Drying shrinkage Drying shrinkage increases with the increase in water content in the mix and lowering of relative humidity. Increase in concreting temperature increases water demand which may lead to increased drying shrinkage. Subsequent cooling from high temperatures, at which the concrete hardens, increases the cracking tendency of concrete.

20. Plastic Shrinkage Prediction Chart.

21. EFFECTS ON SHRINKAGE Drying shrinkage Fig shows the relationship between drying shrinkage and unit water content, for various amounts of cement in the mix. Cement content (or water – cement ratio) has only a secondary importance.

22. PRECAUTIONS/RECOMMENDATIONS FOR CONCRETING IN HOT WEATHER

23. TEMPERATURE CONTROL OF CONCRETE INGREDIENTS Direct approach is to control the temperature of concrete ingredients. The contribution of each ingredient to the temperature of concrete is a function of the Temperature Specific heat, and Quantity used of that ingredient The aggregates and mixing water exert the most pronounced effect on temperature of concrete Thus, all available means should be used for Maintaining these materials at as low temperatures as practicable.

24. TEMPERATURE CONTROL OF CONCRETE INGREDIENTS Aggregates Shading stockpiles from direct sunlight Sprinkling the stockpiles of coarse aggregate with water and keeping them moist When coarse aggregates are stockpiled during hot weather, successive layers should be sprinkled as the stockpile is built up. If cold water is available, heavy spraying of coarse aggregate immediately before use may also be done to have a direct cooling action. Coarse aggregates may also be cooled by Inundating them in cold water or By circulating refrigerated air through pipes.

25. TEMPERATURE CONTROL OF CONCRETE INGREDIENTS Water Mixing water has the greatest effect on temperature of concrete, since it has a specific heat of about 4.5 to 5 times that of cement or aggregate Temperature of water is easier to control than that of other ingredients Even though water is used in smaller quantities than the other ingredients, the use of cold mixing water will effect a moderate reduction in concrete placing temperatures.

26. TEMPERATURE CONTROL OF CONCRETE INGREDIENTS Reduction in water temperature may be most economically accomplished by Mechanical refrigerator Mixing with crushed ice Use of ice as a part of the mixing water is highly effective in reducing concrete temperature On melting alone, ice takes up heat at the rate of 80 kcal/kg. To take advantage of heat of fusion, the ice shall be incorporated directly into the concrete as part of mixing water. But it should be ensure that the ice is completely melted by the time mixing is completed.

27. TEMPERATURE CONTROL OF CONCRETE INGREDIENTS Cement Temperature has a direct effect on the rate of hydration of cement. Change in temperature of cement produces significantly less change in the temperature of fresh concrete than the other ingredients. Cement shall preferably not be used at temperatures in excess of about 77 o C Whenever fresh cement is received it should be allowed to cool before usage Cement silos to be preferably coated with light colored paints (white or yellow)

28. TEMPERATURE CONTROL BY PROPORTIONING OF INGREDIENTS

29. TEMPERATURE CONTROL BY PROPORTIONING OF INGREDIENTS Cement Quantity of cement used in the mix affects the rate of increase in temperature The mix should be designed to have minimum cement content consistent with other functional requirement. Cement with lower heat of hydration should be preferred Cement having greater fineness and high heat of hydration should referably be avoided

30. Water/Plasticizer In hot weather hydration of cement is accelerated This acceleration is generally considered responsible for the increase in water requirement of concrete. Water reducing and set – retarding admixtures should be used to lessen the need for increase in mixing water. Usage of any admixture to be done after ascertaining results of compatibility tests. TEMPERATURE CONTROL BY PROPORTIONING OF INGREDIENTS

31. TEMPERATURE CONTROL OF CONCRETE DURING PRODUCTION AND DELIVERY

32. PRODUCTION AND DELIVERY Temperatures of aggregates, water and cement shall be maintained at the lowest practical level so that the temperature of the concrete is below 40 o C at the time of placement. Mixing time shall be held to the required minimum. The period between mixing and delivery shall be kept to an absolute minimum. Attention shall be given to coordinating the delivery of concrete with the rate of placement to avoid delays in delivery.

33. PLACEMENT AND FINISHING Forms, reinforcement and sub grade shall be sprinkled with cool water just prior to placement of concrete. The area around the work shall be kept wet to the extent possible to cool the surrounding air and increase its humidity, thereby reducing temperature rise and evaporation from the concrete. When temperature conditions are critical, concrete placement may be restricted to the evenings or night. Ample personnel shall be employed to handle and place concrete immediately on delivery. On flat work, all steps in finishing shall be carried out promptly.

34. Concrete shall be placed in layers thin enough and in areas small enough so that the time interval between consecutive placements is reduced and vibration or other working of the concrete will ensure complete union of adjacent portions. Fog sprays applied shortly after placement and before finishing, have been found to be particularly effective in preventing plastic shrinkage cracks. PLACEMENT AND FINISHING

35. Protection and Curing Attention shall be paid to having all surfaces protected from drying. Immediately after surface finish, concrete shall be protected by means of wet (not dripping) gunny bags, hessian cloth. Once the concrete has attained some degree of hardening sufficient to withstand surface damage (approximately 12 hours after mixing), moist curing shall commence. The actual duration of curing in any case shall not be less than 10 days.

36. Continuous curing is important, because volume changes due to alternate wetting and drying promote the development of surface cracking. If possible , water shall be applied to formed surface while forms are still in place. Unformed surfaces shall be kept moist by wet curing. Covering material shall be kept soaked by spraying. Steeply sloping and vertical formed surfaces shall be kept completely and continuously moist prior to and during form removal. Protection and Curing

37. Exposed concrete surfaces, such as pavement slabs, wind is an important factor in the drying rate of concrete. Other conditions being equal, a gentle wind of 15 km/h will cause four or more times as much evaporation from a flat surface has still air. Hence wind breakers shall be provided. On hardened concrete and on flat surfaces in particular, curing water shall not be much cooler than the concrete because of the possibilities of thermal stress and resultant cracking. At the termination of curing wet covers to be removed only after they are completely dry to reduce the rate of drying due to air circulation. Protection and Curing

38. Immersion Ponding Spraying Wet Covering Membrane Curing Curing…………

39. CURING OF CONCRETE ROADS (Curing ……

40. Conclusion For concreting in Hot weather adequate care should be taken for Temperature control of concrete mix ingredients Proportioning of concrete mix Production & Delivery Placement, Protection & Curing Adequate temperature of the concrete at all times Removal of forms / protection Record keeping (temperature/humidity etc) Concrete & concrete specimen testing

41. Concluding Remarks The recommendations apply to more general types of construction such as buildings, bridges, pavements, heavy foundations and other similar structures but in many cases, such as mass concreting for dams etc, precautions given in this code will not be enough and additional precautions will have to be applied. Adoption of these recommended practices will result in concrete possessing improved characteristics in the freshly mixed and hardened state thereby enhancing the durability of concrete & life of structure.

42. T HANK Y OU References - IS : 7861 (Part 1) -1981 (Hot Weather Concreting) SP 23 – 1982 Handbook on Concrete Mixes Concrete Technology by Prof M S Shetty http://www.aboutcivil.com/hot-weather-concreting